The present invention relates to organic electroluminescent devices and more particularly to doping a hole-transport layer and/or an electron-transport layer with suitable color-neutral dopants to improve the operational lifetime of these devices without affecting the electroluminescent color.
A common form of organic electroluminescent device (OLED) includes a substrate and, disposed thereupon, a multilayer structure comprising an anode, an optional hole-injection layer, a hole-transport layer, a luminescent layer, an optional electron-transport layer, and a cathode, each layer of the multilayer structure comprising one or more organic or organometallic compounds. Electroluminescent (EL) devices are attractive because of their low driving voltage, high luminance, wide viewing angle, and capability for full-color flat-panel emissive displays. Tang et al. described this OLED structure in their U.S. Pat. Nos. 4,769,292 and 4,885,211. With the prospect of using OLED in display screens for laptop computers, digital personal organizers, cellular telephones etc., it is highly desirable to improve the operational lifetime of these devices.
The following patents and publications disclose the preparation of OLED with improved operational lifetime. Modifications of the multilayer structure, stabilized cathode materials, and confinement of various carriers and their recombination within the emission zone have achieved significant improvement in the operational lifetime of these devices. So et al., in U.S. Pat. No. 5,853,905, discussed an EL device consisting of a single organic emission layer containing a mixture of electron-transport and hole-transport materials, sandwiched between anode and cathode. However, this device has low efficiency. Popovic et al, in SPIE Conference Proceedings, Vol. 3476, pp. 68-72, 1998, described an EL device with improved efficiency and operational lifetime prepared by mixing an emitting electron-transport material and a hole-transport material in a light-emitting layer. Xie et al., in U.S. Pat. No. 5,989,737, disclosed an OLED in which the hole-transport layer comprises a tertiary aromatic amine doped with a polycyclic aromatic hydrocarbon such as rubrene. This approach has the problem that some dopants, such as rubrene, alter the electroluminescent color of the device.
It is therefore an object of the present invention to provide an OLED with improved operational lifetime. This object is achieved by providing an organic light-emitting device, comprising:
a) a substrate;
b) an anode and a cathode positioned relative to the substrate;
c) an emission layer disposed between the anode and the cathode;
d) a hole-transport layer disposed between the anode and the emission layer; and
e) an electron-transport layer disposed between the cathode and the emission layer,
f) the hole-transport layer comprising at least two sublayers, the sublayer closest to the emission layer containing a color-neutral dopant, and at least one sublayer not containing the color-neutral dopant; and
g) the color-neutral dopant comprising an anthracene derivative.
This object is also achieved by providing an organic light-emitting device comprising:
a) a substrate;
b) an anode and a cathode positioned relative to the substrate;
c) an emission layer disposed between the anode and the cathode;
d) a hole-transport layer disposed between the anode and the emission layer; and
e) an electron-transport layer disposed between the cathode and the emission layer,
f) the electron-transport layer comprising one or more sublayers, at least one sublayer containing a color-neutral dopant; and
g) the color-neutral dopant comprising an anthracene derivative.
This object is also achieved by providing an organic light-emitting device comprising
a) a substrate;
b) an anode and a cathode positioned relative to the substrate;
c) an emission layer disposed between the anode and the cathode;
d) a hole-transport layer disposed between the anode and the emission layer; and
e) an electron-transport layer disposed between the cathode and the emission layer,
f) the electron-transport layer comprising one or more sublayers, at least one sublayer containing a color-neutral dopant,
g) the hole-transport layer comprising one or more sublayers, the sublayer closest to the emission layer containing a color-neutral dopant; and
h) the color-neutral dopants comprising anthracene derivatives.
A color-neutral dopant is a dopant that does not substantially affect the electroluminescent color of the device relative to that of an otherwise identical device from which the dopant is omitted.
The use of a color-neutral dopant permits the electroluminescent color of the present OLEDs to be tuned solely by adjusting the composition of the emission layer. It has been found that certain anthracene derivatives can be used as color-neutral dopants in the hole-transport and/or electron-transport layer of an OLED and that the resulting device has a greater operational lifetime than an otherwise identical device that does not contain the color-neutral dopant.
Other features and advantages of the present invention include the following:
OLEDs with various electroluminescent colors, such as red, green, blue, or white, can be constructed by inclusion of suitable dopants in the emission layer. Thereby, it is possible to construct OLEDs with simultaneously optimized lifetime and chromaticity.
OLEDs of this invention have high luminance efficiency.
OLEDs of this invention can be used with a low-voltage drive source.
The present invention can include the color-neutral dopants in both the hole-transport layer and the electron-transport layer to provide an OLED with greater operational lifetime than would an OLED including the color-neutral dopants into either the hole-transport layer or the electron-transport layer alone.
The devices made in accordance with the present invention have long operational lifetimes and excellent luminance properties.